In the manufacture of semiconductor devices, it is normally necessary to alter the conductivity type of selected regions of the semiconductor body by doping the regions with conductivity modifying impurity atoms. Such doping may be accomplished by various techniques known in the art. One such technique is thermal gradient zone melting (TGZM). This technique can produce very abrupt junctions with unusual configurations and high doping concentrations in a body of semiconductor material in a relatively short period of time. Early descriptions of TGZM and some of its applications can be found in U.S. Pat. No. 2,813,048 issued to W. G. Pfann and in his book, Zone Melting, copyright by John Wiley and Sons, Inc.
Thermal gradient zone melting is a process in which a small amount of dopant material (generally a metal) is deposited on a selected surface of a semiconductor material, such as a semiconductor wafer or ingot, and the semiconductor body is then exposed to a temperature gradient at an elevated temperature. The overall temperature at which the process is carried out must be sufficiently high in order to form a metal-rich liquid zone in the form of a line, droplet or sheet which zone will be established depending on the initial shape or pattern of the deposited metal dopant material. This liquid zone is caused to migrate through the semiconductive material from the cooler surface to the hotter surface leaving in its path a recrystallized region of semiconductor material containing the metal and solid solution within the semiconductor material in a concentration determined by the solubility limit of the metal therein.
The temperature gradient should be uniform and unidirectional if the pattern of dopant material disposed on the entrance surface of the wafer is to be faithfully reproduced in a crystallized dopant zone or region throughout the semiconductive wafer to the opposite surface of the wafer.
One of the most difficult problems relating to the use of TGZM has been the inability of generating a large uniform thermal gradient across the thickness of a thin fragile semiconductor wafer so as to uniformly reproduce the dopant pattern throughout the wafer. Distortion of the thermal gradient tends to be especially prevalent near the edge of the wafer where heat loss is high.
In one technique of TGZM, as disclosed in U.S. Pat. No. 4,001,047, issued to John K. Boah, infrared radiation is employed to produce a thermal gradient through the semiconductor wafer. However, while this thermal gradient is uniform through most of the wafer, due to additional heat loss at the edge of the wafer as compared with the body of the wafer, the thermal gradients are distorted from their otherwise unidirectional direction. This distortion, in turn, causes distortion of the thermal migration pattern of the dopant in the area around the edge of the wafer making this region unsuitable for device fabrication.
For example, in a 2 inch diameter n-type, 10-20 ohm centimeter, silicon (111) wafer, 10 mils thick, the distorted thermal gradient produced by I.R. radiation in accordance with the teachings of Boah extends as far as 0.25 inch into the center of the wafer, causing sideway migration of the liquid zone in this region and reducing the usable area of the wafer by 44%.
In another technique to obtain a uniform thermal gradient throughout the entire semiconductor body, T. R. Anthony and H. E. Cline, have disclosed in U.S. Pat. No. 3,895,967, a method by which thermal gradient distortion can be minimized around the edge of a thick semiconductor ingot as opposed to a thin semiconductor wafer. This method employs a guard ring of semiconductor material of the same thickness as the semiconductor ingot disposed about and spaced from the peripheral edge of the semiconductor ingot. One requirement of this method is that the space or gap between the guard ring and the semiconductor ingot has to be less than 1/10 of the thickness of the semiconductor ingot. If this gap were not maintained, the guard ring became less effective and thermal distortion problems still are present in the peripheral edge portion of the semiconductor ingot. For thin semiconductor wafers, the requirement that the separation between the guard ring and the wafer be less than 1/10 the thickness of the wafer and that the semiconductor wafer and guard ring be coplanar make the use of guard rings commercially unfeasible for a number of reasons.
In another method described by Anthony and Cline in U.S. Pat. No. 4,035,199 the thermal gradient distortion problem was attacked by depositing quarter wave absorption material on selected areas of the surface of the wafer to supply more heat at the edge of the wafer to compensate for the heat loss around the edge and thereby reduce the sideway migration at the edge of the wafer. This method, however, depends on precise thickness control in depositing the quarter wave absorption material and requires the deposition of such material as a separate additional processing step. In addition the heat absorbing layer is only maintained on the surface of the wafer and does not penetrate the depth of the wafer. This process would therefore be relatively costly to implement and difficult to control and would not result in uniform control of heat loss through the thickness of the wafer.
In summary, the present methods of thermal gradient zone melting processing of thin semiconductor wafers either result in a waste of a large portion of the semiconductor wafer or are associated with costly and/or difficult to control processing. Consequently, a method for reducing the thermal gradient distortion problems around the peripheral edge portion of the wafer which does not result in added steps for processing the wafer and is easily controllable is greatly desired.